Immune system response. Artwork representing the activation of a T-lymphocyte cell (bottom right) due to the presence of a foreign cell (top left). The activation requires two signals: the recognition of an antigen (yellow) by a receptor molecule (green) situated on the membrane of the lymphocyte and the release from the foreign body of the substance known as Interleukin-1 (magenta ellipsoids). Due to these messages the T-cell enlarges and starts synthesising RNA, proteins and Interleukin-2 (blue ellipsoids). A red Y-shaped molecule is being released to activate another part of the immune system. T-lymphocytes are white cells which circulate in the blood.